The man behind the 1998 study, Andrew Wakefield, MD, continues to defend it. But 10 of his co-authors have repudiated it. Last year it was formally retracted by The Lancet. And after a months-long hearing, Wakefield and his senior research advisor had their medical licenses revoked for unethical treatment of patients.

But now a lengthy investigation by U.K. investigative reporter Brian Deer finds that Wakefield deliberately faked the study. Deer's findings, first published in the Sunday Times, now appear in BMJ -- accompanied by a scathing editorial by BMJ editors Fiona Godlee and colleagues.

"Deer unearthed evidence of clear falsification," the editorial says. "Who perpetrated this fraud? There is no doubt that it was Wakefield's."

Despite the fact that it involved only 12 patients, the Wakefield study had a huge effect. MMR vaccination rates plummeted in the U.K., Europe, and parts of the U.S. Wakefield continues to have a following among parents who believe, in spite of strong medical evidence to the contrary, that vaccination is a major cause of autism.

The entire affair raises a number of questions. Here is WebMD's FAQ:

Why is the 1998 Wakefield study important?

Children often exhibit the first unmistakable signs of autism when they are toddlers -- an age at which they are receiving their childhood vaccination series. Moreover, some children display regressive autism: They seem normal, but then dramatically lose the ability to speak and to relate to others.

By 1998, a number of parents became convinced that their children's autism was caused by the MMR vaccine. They hired lawyers to sue vaccine makers for damages. But there was little scientific evidence linking the vaccine to autism.

Wakefield's study was the first to suggest a plausible link between MMR vaccination and autism. The study suggested that the vaccine caused a gastrointestinal syndrome in susceptible children, and that this syndrome triggered autism.

The study purported to look at a series of 12 children treated consecutively at a large London hospital. Wakefield and colleagues reported that all 12 children had intestinal abnormalities and developmental regression beginning one to 14 days after MMR vaccination.

Despite the small size of the study, it led to widespread fear of the MMR vaccine. Measles once again became endemic in the U.K. and in other European nations.

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What was wrong with the Wakefield study?

Details of the medical histories of all the children in the Wakefield study were made public in an investigation by the U.K. General Medical Council. Deer also interviewed several of the parents whose children were in the study.

Here are some of the major problems with the study, as laid out by Deer in BMJ:

The children in the study were not randomly selected. None of them lived anywhere near the hospital where Wakefield's team examined them. One came from as far away as California. All were recruited through anti-MMR-vaccine campaigners.

Wakefield did not disclose that he was acting as a paid consultant to a U.K. lawyer who was suing MMR vaccine makers for damages. Wakefield was paid about $668,000 plus expenses.

Despite being described as "previously normal," five of the children had evidence of developmental problems before receiving the MMR vaccine.

Only one of the 12 children in the study had regressive autism, although the study reported that nine of them had this condition. Three of these nine children were never diagnosed with autism.

In nine cases, gut examinations of the children were changed from "unremarkable" to "non-specific colitis."

For all 12 children in the study, medical records and parent accounts contradict case descriptions in the published study.

The BMJ editors conclude that these discrepancies show that Wakefield deliberately faked the study.

"Is it possible that he was wrong but not dishonest: that he was so incompetent that he was unable to fairly describe the project or to report even one of the 12 children's cases correctly?" they ask. "No. A great deal of thought and effort must have gone into drafting the paper to achieve the results he wanted."

What is Wakefield's explanation?

Wakefield did not reply to WebMD's interview request in time for publication. In an interview with CNN's Anderson Cooper, he denied any wrongdoing.

Of the BMJ articles, he said, "It is a ruthless, pragmatic attempt to crush any investigation into vaccination concerns."

Deer, he told Cooper, "is a hit man. He has been brought in to take me down."

Wakefield claims Deer is in the pay of pharmaceutical companies, although Deer reports funding only from the Sunday Times of London and U.K.'s Channel 4 television network. His BMJ reports were funded by the journal.

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Does this have anything to do with thimerosal or mercury in vaccines?

No. Thimerosal is a mercury-based preservative. It cannot be used in live-virus vaccines such as the MMR.